Methods :
The expression of CAII, III, IV, XII and XIV in rabbit corneal endothelium and kidney was examined by quantitative RT-PCR. To test CA activity, rabbit cornea was superfused on the endothelial surface with Glutathione Bicarbonate Ringer (GBR saturated with 5% CO2), or supplemented with a general CA inhibitor, acetazolamide. Perfusate was collected over 30 minute intervals and assayed for [lactate]. Corneal thickness was measured every 15 min by OCT imaging. At the end of the perfusion, the lactate in the cornea was measured.

Results :
The expression level of CAs was CAII >> CAIII > CAXIV > CAXII > CAIV in the corneal endothelium, and was CAII >> CAXII > CAIV > CAIII in the kidney. Whereas CAXIV was undetectable in the kidney. By comparison to CAIV, CAII was 1.5x105, CAIII 57, CAXIV 12 and CAXII 3.6 times greater in corneal endothelium. In mounted and perfused rabbit corneas, corneal thickness in standard GBR perfusion resulted in 4.6±1.73 µm/hr increase. Acetazolamide (0.1 mM) led to a maximum corneal swelling rate of 8.9±1.15 µm/hr. Lactate efflux was 467±8.27 nmols/hr in GBR, and reduced to 391±15.68 nmols/hr by acetazolamide. Lactate retention in the cornea after a 5-hour perfusion with GBR was 11.97±1.33 nmols/mg dry cornea, and 17.63±3.08 nmols/mg dry cornea treated with acetazolamide.

Conclusions :
1) Carbonic anhydrase II is prominently expressed in the corneal endothelium. 2) The unique expression of the transmembrane CAXIV with an extracellularly located active site in the corneal endothelium may have specific interest. 3) Carbonic anhydrases contribute to the maintenance of corneal hydration because they facilitate the transmembrane flux of lactate:H+, which is tightly coupled to fluid movement.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.